Exhaust gas cleaning catalysts whose catalytic component is supported on a honeycomb structure or a filter have been widely used for cleaning the exhaust gases from diesel engines or gasoline engines. For example, in the case that an exhaust gas contains nitrogen oxides, an exhaust gas cleaning catalyst is used which oxidizes nitrogen monoxide in nitrogen oxides to nitrogen dioxide. And it has been known that such nitrogen dioxide assumes the role of oxidizing the soot etc. in the exhaust gas when reduced to nitrogen. A precious metal such as platinum, palladium or rhodium, or the oxide thereof can be used as a catalytic component for such an exhaust gas cleaning catalyst and platinum has been particularly often used.
Exhaust gas cleaning catalysts whose catalytic component is platinum can be produced by a process in which a solution containing a platinum compound is brought into contact with a carrier. Specifically, platinum can be supported on a carrier by a process in which a solution containing excess platinum is brought into contact with a carrier, dried, followed by baking to force platinum to deposit on the carrier, or by a process in which a carrier is impregnated with a solution that contains platinum in an amount equal to or more than the amount of saturated adsorption of the carrier to allow the carrier to adsorb platinum until equilibrium is established.
Patent Document 1 discloses an exhaust gas cleaning catalyst which is used for the cleaning of nitrogen oxides and whose catalytic component is platinum and carrier is γ-alumina. In this exhaust gas cleaning catalyst, platinum is supported on γ-alumina by impregnating γ-alumina with an aqueous solution of chloroplatinic acid, drying at 100° C. for 12 hours, followed by baking at 500° C. (Patent Document 1, Preparation Example 1).
Patent Document 1: Specification of Japanese Patent No. 3791968